I worked as a PM for a while. 1st biggest difference is you don't call the show. 2nd is you deal a lot with the budget...specifically build and crew. You tend to monitor that portion of it where as the Company Manager deals with actor budget. As a PM you aren't constantly in rehearsal, but you are generally at your desk or in the shop - ordering something, setting up various meetings, trying to get the best deal with the budget you have, etc.
You check in on rehearsals to make sure the SM has what he or she needs, PM generally hires crew as well and that is also part of your budget. You also stay on top of the design schedule and work out at least part of those contracts. PM tends to look at the big picture for the entire season where as the SM is focusing on the current show in rehearsal or on stage and maybe even the next show, but that's it.
I'll tell you, Admin was my least fave part of being an SM. I loved calling the shows and being in rehearsal. If you love the paperwork side of things and numbers, Production Management is where you want to be.
